Commonly images represented by a digital signal such as medical images are subjected to image processing during or prior to displaying or hard copy recording.
The conversion of grey value pixels into values suitable for reproduction or displaying may comprise a multi-scale image processing method (also called multi-resolution image processing method) by means of which the contrast of the image is enhanced.
According to such a multi-scale image processing method an image, represented by an array of pixel values, is processed by applying the following steps. First the original image is decomposed into a sequence of detail images at multiple scales and occasionally a residual image. Next, the pixel values of the detail images are modified by applying to these pixel values at least one conversion. Finally, a processed image is computed by applying a reconstruction algorithm to the residual image and the modified detail images. Such a method is illustrated in FIGS. 2 and 3.
There are limits for the behavior of the conversion functions. Grey value transitions in the image can be distorted to an extent that the appearance becomes unnatural if the conversion functions are excessively non-linear. The distortions are more pronounced in the vicinity of significant grey level transitions, which may result in overshoots at step edges and loss of homogeneity in regions of low variance facing strong step edges. The risk of creating artifacts becomes more significant for CT images since they have sharper grey level transitions, e.g. at the interface of soft tissue and contrast media. One has to be careful using the multi-scale techniques on CT images.
In U.S. Pat. No. 6,731,790 B1 a method is described in which artifacts are corrected in an image that is represented by a digital signal. The corrected artifacts originate from any kind of non-linear modification of the components of a multi-scale image representation.
A gradient representation of the original image is generated and modified. A modification step is applied to gradient images.
A reconstruction process is applied to the modified gradient representation.